ESO Astronomical Glossary - S

Satellite Satellites are objects that orbit a planet or other solar system body. Many man-made satellites and one natural satellite (the Moon) orbit the Earth.

Saturn Saturn is the sixth planet from the Sun. It is the second-largest planet in our solar system. It is best known for its distinctive rings, which are made of ice chunks that range in size from the size of a fingernail to the size of a car. Saturn also has many satellites, including Titan, the second largest moon in the solar system.

Seeing The term seeing in astronomy is used to describe the disturbing effect of turbulence in the Earth's atmosphere on incoming starlight. The study and characterisation of seeing at a given location is an important part of the site selection process for astronomical observatories.

Solar mass A solar mass is the amount of mass in our Sun; it is also the unit in which the masses of other stars, galaxies, and other large celestial bodies are expressed. The solar mass is 1.99 x 1030 kg, which is about 330,000 times the Earth's mass.

Southern Cross A small but very distinctive constellation in the southern sky. The Southern Cross inspired many ancient cultures because of it prominence in the night sky. The VLT third Unit Telescope is named after the Southern Cross in the Mapuche language, "Melipal".

Spectral classification System whereby stars are classified according to features in their spectra. The most common system is called the MK system, after astronomers Morgan and Keenan who devised the system in 1943. The basic framework of the MK system is that each star is classified using a letter to indicate its temperature, a number from 0 to 9 to further subdivide the temperature scale, and a roman numeral to describe its luminosity class. The letter sequence in order of descending temperature is O-B-A-F-G-K-M. Further classes have also been devised outside the traditional MK sequence, for example to describe cool brown dwarfs (classes L and T).The luminosity classes go from I (supergiant) to V (dwarf). The Sun has spectral type G2V.

Spectrograph (spectrometer) Spectrometers and spectrographs are instruments that disperse the light from astronomical sources into its spectrum and images it for analysis. See also spectroscopy.

Spectroscopy Spectroscopy is the study of spectra of astronomical objects.

Spectrum In astronomy, the spectrum of an astronomical object is the rainbow of electromagnetic radiation emitted by the object, separated into its constituent wavelengths. The study of astronomical spectra can give astronomers information on the chemical composition of the source, as well as their redshift, magnetic field, and many other properties. The study of spectra is known as spectroscopy.

Speed of light The speed of light, often denoted 'c', is the fastest possible speed in a vacuum, equivalent to 300 000 km per second.

Spiral galaxy Spiral galaxies are galaxies with a central, dense area and pinwheel-shaped arms spiralling outwards around it. The Milky Way and M31 (commonly known as the Andromeda Galaxy) are both spiral galaxies.

Star A star is a self-luminous sphere of hot gas, predominantly hydrogen, held together by gravity; ordinary stars generate energy by nuclear fusion reactions in their dense cores, which lead to the production of helium and heavier elements. Stars are formed and reside in galaxies. They are generally classified according to their temperature and luminosity.

Stellar evolution The term stellar evolution refers to the life cycle of stars.

Submillimetre astronomy Submillimetre astronomy is the branch of astronomy that studies radiation in the submillimetre part of the electromagnetic spectrum. The Atacama Large Millimeter/submillimeter Array (ALMA) became the world's biggest submillimetre observatory on its completion in 2013.

Supernova A supernova is a huge explosion that occurs at the end of a heavy star's life. A supernova releases a tremendous amount of energy, expelling the outer layers of the star and momentarily becoming brighter than the entire galaxy it resides in. Alternatively a supernova explosion is thought to occur when a white dwarf has accreted sufficient material from a companion (see cataclysmic variable).